A series of experiments with the following objectives is proposed: 1. To determine antibiotic additions which will produce maximal reductions in amino acid degradation consistent with minimal interference with the general metabolic activity of ruminal microorganisms in vitro. 2. To determine optimal dietary antibiotic conditions (compatable with total nutrition) to maximize rumen by-pass of free amino acids in calves as indicated by plasma amino acid concentrations after oral administration of fixed doses of amino acids. 3. To quantitate the rumen by-pass of supplemental amino acids in cattle fed antibiotics at recommended feed additive levels and at optimal levels as determined by the plasma response studies. 4. To evaluate possible "protein-sparing" effects of antibiotics by studying protein and antibiotic interactions in feedlot cattle.